docs: added reference to DRF-gis in fields

added django-rest-framework-gis to third party packages section in /docs/api-guide/fields.md

docs/api-guide/fields.md

@@ -361,6 +361,9 @@ The [drf-compound-fields][drf-compound-fields] package provides "compound" seria
 
 The [drf-extra-fields][drf-extra-fields] package provides extra serializer fields for REST framework, including `Base64ImageField` and `PointField` classes.
 
+## django-rest-framework-gis
+
+The [django-rest-framework-gis][django-rest-framework-gis] package provides geographic addons for django rest framework like a  `GeometryField` field and a GeoJSON serializer.
 
 
 [cite]: https://docs.djangoproject.com/en/dev/ref/forms/api/#django.forms.Form.cleaned_data
@@ -370,3 +373,4 @@ The [drf-extra-fields][drf-extra-fields] package provides extra serializer field
 [iso8601]: http://www.w3.org/TR/NOTE-datetime
 [drf-compound-fields]: http://drf-compound-fields.readthedocs.org
 [drf-extra-fields]: https://github.com/Hipo/drf-extra-fields
+[django-rest-framework-gis]: https://github.com/djangonauts/django-rest-framework-gis